Distinct immune properties of the N- and C-termini of the immunosuppressive domain of Ebola virus glycoprotein.

Ebola virus (EBOV) causes a severe human disease with high lethality. Pathogenesis of EBOV disease is characterized by a paradoxical combination of hyperinflammation and immunosuppression. EBOV has a single envelope glycoprotein (GP), which is a type I transmembrane protein with strong immunomodulatory effects. GP contains a conserved immunosuppressive domain (ISD) with a high similarity to ISDs of envelope proteins retroviruses. To investigate the effects of ISD, a set of 17 EBOV viral-like particle (VLP) constructs containing the entire EBOV nucleoprotein, VP40, and GP with single alanine or glycine substitutions in each position of ISD was generated and tested in human peripheral blood mononuclear cells (PBMCs). Wild-type VLPs induced inflammatory responses; however, when added to pre-stimulated cells, they reduced inflammation, thus exerting immunosuppressive properties. Substitution of lysine at ISD position 5 (Lys-5) increased anti-inflammatory properties by reducing proliferative responses of VLPs and also reducing nuclear factor of activated T-cells 1 (NFAT1) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation. In contrast, substitution of tryptophan at position 14 (Trp-14) increased both the pro-inflammatory and the proliferative responses and the adhesion of VLP-infected human monocytes to microvascular endothelial cells. Thus, the ISD N- and C-termini have pro- and anti-inflammatory properties, respectively, suggesting their unique implications in the EBOV pathogenesis. Furthermore, the immunomodulating effects of ISD were also mediated by shed GP, which is abundant in the medium. These data may be useful for the development of treatments for diseases caused by EBOV by targeting the ISD.IMPORTANCEOur data suggest that the ISD N-terminus plays a role in activating immune cells and pro-inflammatory response. In contrast, the C-terminus of ISD downregulates the pro-inflammatory response through the reduction of NF-kB and NFAT activities. The data also show that EBOV GP increases the adhesion of monocytes to endothelial cells, and the effect is inhibited by the ISD C-terminus. Moreover, the data demonstrate that the immunomodulating effects of ISD are mediated not only by the virus-associated GP but also by the shed GP, which is abundant in the medium. Pathogenesis of the disease caused by EBOV is characterized by hyperinflammation and some features of immunosuppression, which could in part be affected by the complex effects of the ISD. These data indicate that targeting the ISD may be considered for the development of treatments for the disease caused by EBOV.